The Metropolitan Police, which is leading the counter-terrorism investigation, has not publicly identified anyone it has wanted to speak to, but occasionally that type of information can be withheld for operational reasons.

In this case Mr Sedwill's response suggested police simply don't yet have a firm suspicion that identifies anyone who may have been involved.

The Government has blamed Russia for the attack, claims Vladimir Putin and other Russian officials have vigorously denied.

Image:The area around the bench where the Skripals were found is being decontaminated

Mr Sedwill also confirmed the review of security around individuals in the UK who may be at risk of a similar attack.

He said: "The police who are responsible for protective security and the various agencies alongside them are reviewing the security of all people who might be vulnerable in that way."